A. Field of the Invention
The present invention relates to methods and apparatus for thermal regulation of artificially infused intravenous fluids.
B. The Prior Art
In surgery and after extreme trauma, patients receive intravenous fluids at high rates. Flow rates typically range between 40 and 2000 milliliters per hour and sometimes more. Typically, the fluid temperature upon entering the body is 20 degrees centigrade or cooler. Prior to infusion or transfusion the solutions are refrigerated to prevent incubation of harmful organisms in the fluid media. Upon infusion into the body, the intravenous fluid temperature increases to body temperature (37 degrees centigrade). The heat energy required for this essential temperature increase is supplied by an increased metabolic rate in the patient. This thermal load is not trivial when compared to normal metabolic processes in humans. As a baseline, 2000 food calories are consumed daily. This corresponds to a constant power consumption of 96.6 watts. Intravenous fluid, at twenty degrees centigrade, being infused at a rate of 2000 cubic centimeters per hour is equivalent to a thermal power load of 39.5 watts.
Thermal loading in this case is 41% of the baseline power consumption at 2000 food calories per day. Clearly, infusion induced thermal loading can be a principal contributor to the onset of thermal shock and possibly patient mortality. Additionally, such thermal loading may mask the onset of infection during early postoperative care. Presently there is no temperature control device in general widespread use in hospitals and trauma centers.
For purposes of this application the terms "intravenous fluid(s)""IV solution" or "IV fluid" will be used interchangeably and will be used to generically refer to bodily fluids that may be infused or transfused into the body. Blood or blood components are the most typical form of such intravenous ("IV") fluid, however, the term should also be understood to refer to the multitude of artificial solutions or additives that are injected, infused or transfused into the arterial or venous system of patients during medical treatment.
An obvious method for heating an IV fluid is by placing the standard intravenous fluid bag into a warmer where the fluid would reach body temperatures and remain there with a high degree of stability. This method would be ideal if the intravenous fluid were not an ideal incubation media. However, such a preheating method would promote the growth of bacteria, fungi and other temperature sensitive organisms and is medically unacceptable.
Another method would be to employ a counterflow heat exchanger working on the principle of logarithmic mean temperature difference. This method would require a separate working fluid reservoir to be heated, a fluid pump, and a long heat exchanger. The method is not suitable for the rapid feedback required in a trauma center. Temperature control and flexibility are limited by the size of the working fluid reservoir. At best, the fluid reservoir size would equal the intravenous flow tube size. Even at this stage, a counterflow device is over complicated and poorly suited to the specified task. Additionally, this method does not satisfy the requirement of compactness and simplicity. The intravenous tubing could be run through a large waffle iron geometry heat exchanger. Such a device would work in the steady state flow mode with little feedback involved. However, rapid changes in flow rate and the need for precise control in the variable flow regime rule out this method. The waffle iron geometry would also be too large and time consuming in practice. The ideal heat exchanger must strongly couple the electrical input power directly to the intravenous fluid.
In designing the apparatus and method to effectively and efficiently regulate the temperature of IV fluids as they enter the human body certain engineering design parameters were developed and maintained. The device must not store a large amount of heat energy so that the feedback controls can rapidly adjust to variable fluid flow. Additionally, the heat exchanger had to be small and lightweight. Feedback sensors must monitor the flow rate, the initial fluid temperature, and the final intravenous fluid temperature before it enters the body. Any system must be able to detect a sudden flow shutoff. This prevents overheat in active feedback control systems and "tube" incubation in passive systems. A further constraint of an ideal system would be that the surface temperature of any heating element is maintained at or below 45.degree. C. Red blood cells within the IV fluid which are exposed to temperatures above 45.degree. C. may be damaged. Design of the heat exchanger should minimize "hot spots" along the length of any heating element for a given flow rate.
Prior art patents that have unsuccessfully addressed the identical problem include U.S. Pat. No. 1,794,215; U.S. Pat. No. 2,124,293; U.S. Pat. No. 4,038,519; and U.S. Pat. No. 4,384,578.
Specifically, in U.S. Pat. No. 1,794,215 to Titus a device for the intravenous injection of medicated solutions is disclosed. The device includes a heating device made of glass and shaped to provide a conduit through which the medicated solution flows. A heating element formed of materials such as copper wire is wound around the wall of the conduit. Surrounding the outer wall and in spaced relation thereto is an outer wall adapted to enclose the heating device throughout the major portion of its length.
U.S. Pat. No. 2,124,293 to Goldstein discloses an infusion apparatus for injecting fluids into the human body and more specifically a heating device therefor. The heating apparatus of Goldstein consists of an inner tube and a separate outer tube. The outer tube comprises a supporting tube of suitable material such as rubber, upon which a heating coil wound in the form of a helix is found. The heating coil is covered by a layer of heat insulated material such as asbestos which may in turn be covered by a rubber casing.
U.S. Pat. No. 4,038,519 to Foucras discloses a flexible heating tube for medical use. The heating tube includes a flexible pipe of transparent plastic material which is provided with at least one electrical helical resistance heating conductor and at least one helical filiform temperature measuring resistance probe. The two elements are wound on the same axis and are embedded in the wall of the flexible pipe and are surrounded in relationship to the bore of the pipe.
Finally, U.S. Pat. No. 4,384,578 to Winkler discloses a biomedical flow sensor which includes a resistor type heater on the upstream side of a metal contact shell used to heat a solution. Other patents identified as being of general interest include:
______________________________________ 3,374,066 to Farrant 4,525,163 to Slavik et al. 3,768,977 to Brumfield et al. 4,532,414 to Shah et al. 4,065,264 to Lewin 4,576,182 to Normann 4,073,622 to Luppi 4,585,056 to Oscarsson 4,138,464 to Lewin 4,612,170 to Luther et al. 4,160,801 to Badolato et al. 4,622,140 to Lee et al. 4,177,816 to Torgeson 4,623,333 to Fried 4,231,425 to Engstrom 4,648,865 to Aigrer 4,451,562 to Elgas et al. 4,705,505 to Fried 4,464,563 to Jewett ______________________________________
As mentioned hereinabove, the apparatus and method of this invention have application in any medical or biomedical applications wherein an IV fluid is to be injected, transfused or otherwise artificially placed in the human body. Typical clinical situations in which the fluid heating apparatus of this invention will have specific utility are: trauma patients, patients in septic shock, patients with localized injuries, medical procedures wherein maintenance of basic metabolic rates are critical, and health care of compromised patients. In medical trauma the patient typically has a massive loss of blood, plasma and other body fluids. The patient is typically in shock and the patient's body temperature is typically already depressed. The shock is secondary to fluid volume depletion. It is important that any transfused fluids be incorporated into the body at body temperature to avoid a secondary shock caused by the temperature adjustment required by the body.
An example of septic shock is when a patient has peritonitis resulting from a ruptured internal organ. Typically an infection sets in and the body fluids "third space" meaning that the blood volume drops as plasma fluids swell the walls of the intestines and other abdominal organs. It is not uncommon that three to six liters of blood plasma can be lost. In replenishing the lost body fluids temperature maintenance of the fluids entering the body at close to normal body temperature is critical. Requiring metabolic adjustment of the internal body fluids may deepen trauma or cause other physiologically undesirable events to occur.
Localized injury is another example where plasma fluids swell the damaged area of the body. The fluid loss can be as much as 1.5 liters for a broken hip in the elderly or related types of injury. Lesser amounts are encountered with other injuries. The temperature sensitive replacement of body fluids in these trauma situations is critical.
It is important in many medical operational procedures to maintain the basic metabolic rate. A caloric food consumption of 2,000 food calories per day represents an average power consumption of 96.5 watts. In compromised patients body temperature can drop to 35.5.degree.-34.degree. C. or lower. If IV fluids are infused at 36.degree. to 37.degree. C. the body does not have to expend energy to warm the fluids. Energy can be used to fight infection or implement the restorative processes. Additionally, the infusion of cold fluids may actually cool patients. This effect could mask a temperature rise during the onset of severe infection and therefore delay critical treatment or medication.
Finally, the method and apparatus of this invention have shown utility in the treatment of compromised patients. Compromised patients are those patients described as the elderly, those with other diseases and metabolic problems, and immune compromised individuals such as aids patients, chemotherapy patients or radiation therapy patients; and finally, compromised patients include those with malignant diseases. The use of thermally sensitive materials and the transfusion of thermally sensitive materials is made possible through the use of the apparatus and method of this invention. The medical benefits vary from convenient to critical.